The present invention relates to an image forming apparatus, particularly, to an image forming apparatus using a liquid developing agent.
A wet image forming method using a liquid developing agent such as an electrophotographic recording method or an electrostatic recording method produces various merits that cannot be achieved by a dry image forming method and, thus, the wet image forming method is being reevaluated in recent years.
To be more specific, the wet image forming method is advantageous over the dry image forming method in that a high image quality can be achieved because it is possible to use very fine toners of a submicron order in the wet image forming method, that the wet image forming method is economical because a sufficiently high image density can be achieved with a small amount of the toner; that a texture fully comparable with that of a printing, e.g., offset printing, can be achieved with a small amount of the toner, and that the energy saving can be achieved because the toner can be fixed to a paper sheet at a relatively low temperature.
On the other hand, several essential problems remain unsolved in the conventional wet image forming method using a liquid toner, with the result that the dry image forming method has been employed substantially exclusively over a long period of time. One of the problems inherent in the wet image forming method resides in the transfer step.
The first problem in the transfer step is deterioration of the image quality. Specifically, in the conventional transfer step, the developing agent attached to the photoconductor is directly transferred onto a paper sheet under the action of an electric field. As a result, the developing agent was transferred nonuniformly onto the paper sheet in accordance with the fluctuation of the electric field conforming with the irregularity on the surface of the paper sheet. Also, a defective transfer of the developing agent tended to take place depending on the nonuniformity in the electrical characteristics of the paper sheet and also depending on the environment. Because of these problems, the quality of the transferred image was markedly deteriorated.
As a measure for solving these problems, proposed is a method in which a toner image is temporarily transferred from a photoconductor onto an intermediate transfer medium, followed by further transferring the toner image onto a paper sheet. To be more specific, a method of transferring a toner image from a photoconductor onto an intermediate transfer medium under the action of an electric field, followed by further transferring the toner image from the intermediate transfer medium onto a paper sheet under pressure (and heat), is disclosed in, for example, U.S. Pat. Nos. 5,148,222, 5,166,734 and 5,208,637.
Also, a method, in which a toner image transfer onto an intermediate transfer medium and onto a paper sheet is carried out under pressure (and heat) without employing a toner image transfer under an electric field, is disclosed in, for example, Japanese Patent Publication (Kokoku) No. 46-41679 and Japanese Patent Disclosure (Kokai) No. 62-280882. Since it is relatively easy for the intermediate transfer medium to be formed of a material having a smooth surface and low in nonuniformity and fluctuation in electrical resistance, the method disclosed in these prior arts makes it possible to drastically improve the deterioration in the image quality caused by the toner image transfer, compared with the case where the toner image is directly transferred onto a paper sheet under the action of an electric field.
Deterioration in the image quality can also be markedly suppressed in the case where the toner image is transferred onto the intermediate transfer medium under pressure and heat. Also, in these proposals in which the toner image is transferred onto the paper sheet under heat and pressure, it is possible to eliminate the problems observed in the toner image transfer under the action of an electric field such as a nonuniform transfer caused by fluctuation of the electric field depending on the irregularity on the surface of the paper sheet and a defective transfer depending on the nonuniformity of the electric characteristics of the paper sheet and on the environment.
However, practical problems remain unsolved in these proposals, as pointed out below. First of all, since the intermediate transfer medium is used, the image forming process is rendered complex. Also, the image quality is fluctuated in accordance with deterioration of the intermediate transfer medium. Since the intermediate transfer medium is generally required to exhibit a sufficient elasticity and satisfactory release characteristics, the intermediate transfer medium comprises in many cases an elastic layer made of, for example, rubber and a release layer consisting of a silicone-series resin or a fluorine-containing resin and formed on the surface of the elastic layer. Because of the particular construction, the intermediate transfer medium is inferior in durability to the other constituents of the apparatus.
Further, it is difficult to maintain 100% of the transfer efficiency in the step of transferring the toner image from the intermediate transfer medium onto the paper sheet, making it necessary to use a cleaner for removing the toner remaining on the intermediate transfer medium after transfer of the toner image onto the paper sheet. Use of the cleaner makes the image forming system more complex in construction. In addition, the durability of the intermediate transfer medium is further deteriorated by the damage done by the cleaner to the intermediate transfer medium.
As a measure for overcoming the above-noted problems inherent in the intermediate transfer medium, proposed is a method of directly transferring the toner image from the photoconductor onto the image carrier (paper sheet) under heat and pressure. For example, disclosed in U.S. Pat. No. 5,608,507 is a method of directly transferring an image of a liquid toner from a photoconductor having a release layer on the surface onto a paper sheet under heat and pressure.
In the invention disclosed in the U.S. Patent noted above, the liquid toner image attached to the latent image on the surface of a photosensitive body is dried out completely and, then, transferred onto the paper sheet. As a result, the solvent is not attached to the paper sheet, making it possible to suppress the release of a harmful solvent vapor to the outside of the image forming apparatus.
However, according to the experiment conducted by the present inventors in accordance with the method disclosed in the particular U.S. Patent, it has been clarified that the method disclosed in the U.S. Patent is accompanied by the following problems. First of all, it is very difficult to transfer the toner image, in which the solvent has been completely dried, onto a paper sheet under pressure (and heat). As described in detail in the U.S. Patent, in order to improve the transfer efficiency, it is necessary to form a photosensitive layer on an elastic lining layer so as to improve the bonding strength between the surface of the paper sheet and the toner by utilizing the elastic deformation of the photosensitive layer.
In this case, it has been found that the photosensitive layer is deformed repeatedly in accordance with deformation of the elastic body under pressure produced by the pressurizing member pushing the back surface of the paper sheet. As a result, a fatigue breakage takes place in the binder resin constituting the photosensitive body, leading to a marked shortening in the life of the photosensitive body. It should be noted in this connection that the photosensitive body is required to be flexible, making it impossible to use a metallic photosensitive body made of, for example, an amorphous silicon or selenium. In other words, it is necessary to use a photosensitive body containing a binder resin such as an organic photosensitive body. It follows that it is difficult to improve the life of the photosensitive body.
It should also be noted that, in order to obtain a good transfer efficiency, it is necessary for the surface layer of the photosensitive body to be formed of a material having markedly high release characteristics. In short, it is very difficult to meet both excellent photosensitive characteristics and high release characteristics. Further, in order to obtain a high transfer efficiency close to 100%, it is necessary to heat the photosensitive layer and the pressurizing member (back up roller) to a temperature not lower than 100.degree. C. so as to melt the toner sufficiently and, at the same time, to push the paper sheet against the photosensitive body at a high pressure. Naturally, the photosensitive body is further deteriorated by the heating. Also, a large amount of energy is required and the required driving torque is increased.
As described above, if it is intended to obtain a good transfer efficiency in the conventional wet image forming method of a direct image transfer type utilizing heat (and pressure), the life of the photosensitive body is shortened and it is difficult to select the suitable material. In addition, a large heat energy is required, and the required driving torque is increased.